The present invention relates to a method of and its corresponding device for switching from a first compressed data input stream to a second compressed data input stream, resulting in a compressed data output stream.
Such an invention can be useful, for example, for switching and editing MPEG compressed video signals.
International patent application WO 99/05870 describes a method and device of the above kind. This patent application relates, in encoding/decoding systems, to an improved method of switching from a first encoded video sequence to a second one. In order to avoid underflow or overflow of the decoded buffer, a transcoding of the input streams is used to shift the temporal position of the switching point and to obtain at the output of the transcoders, streams containing an identical entry point and the same decoder buffer characteristics.
The previously described method has several major drawbacks. According to the background art, the output bit rate of each transcoder is equal to its input bit rate, which makes the switching method not very flexible. Moreover, said method implies that the first picture of the second video sequence just after the switch will be an Intra-coded (I) picture.
Finally, the solution of the background art is rather complex and costly to implement as the switching device needs two transcoders.
It is an object of the invention to provide a method of switching and its corresponding device that is both flexible and easy to implement.
To this end, the invention relates to a switching device as described in the field of the invention and comprising:
a buffer system intended to store the data contained in the first and second input streams,
control means intended to control the storage of the input streams in the buffer system in order to switch, at a switch request, from the first input stream to the second input stream using a commutation device,
and a transcoding system intended to provide the output stream in a seamless way from the output of the commutation device.
The present invention allows to switch from a first compressed data stream encoded at a bit rate R1 to a second compressed data stream encoded at a bit rate R2, the output stream resulting from the switch being encoded again, using the transcoding system, at a bit rate R where R may be different from R1 and R2. Thus, such a switching device has a flexible behavior.
The switching device according to the invention is also characterized in that:
the buffer system comprises a first buffer and a second buffer intended to store the data contained in the first and the second input stream, respectively,
the transcoding system comprises one transcoder,
the commutation device is controlled to switch from the output of the first buffer to the output of the second buffer when said first buffer has transmitted a set of M pictures of the first input stream, said second buffer being controlled by the control means to transmit an I picture,
and said switching device comprises means for generating B pictures without forward predictions for a set of M pictures of the second input stream including said I picture.
As this switching device uses only one transcoder, its implementation will be less complex and less expensive.
Finally, the switching device according to the invention is characterized in that:
the buffer system comprises a first buffer and a second buffer intended to store the data contained in the first and the second input stream, respectively,
the transcoding system comprises, in association with each input stream, first means for decoding and second means for decoding,
the commutation device is controlled to switch from the first input stream after decoding by the first means to the second input stream after decoding by the second means when the first buffer has transmitted a set of M pictures of the first input stream, the second buffer being controlled by the control means to transmit an I picture or a P picture, which is re-encoded as an I picture using decoding-encoding means,
and said switching device comprises means for generating B pictures without forward predictions for a set of M pictures of the second input stream including said I picture.
Such a switching device allows to switch to a second compressed video stream that is starting with a P picture. Thus, the flexibility of the system is increased.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.